Comets that are visible to the naked eye, without binoculars or telescopes, are like tax windfalls or playoff appearances by favorite sports teams. They only happen once every five or 10 years if you're lucky.

But this year, there may be two.

One of them is racing across the sky this week. Named Pan-STARRS, the comet was discovered in June 2011 by astronomers working atop the Haleakala volcano in Maui using an array of cameras and telescopes known as the Panoramic Survey Telescope and Rapid Response System, or Pan-STARRS.

The comet is expected to be visible all week, astronomers say, flying low in the western horizon. The good news is that people wanting to see it don't need to stay up all night. The best time for viewing is 30 minutes after sunset, with sunset ranging from about 7:10 p.m. to 7:15 p.m. this week.

The bad news? Because the viewing time is so close to sunset, the glow of twilight may make it hard to see. And if you haven't seen it within about one hour of sunset, you've lost your chance until the next night.

"If you look where the sun is setting, look at little bit to the left," said Michael Packer, a spokesman for the San Jose Astronomical Association, a nonprofit stargazers club. "It will look like a fuzzy star, with a tail. It should be obvious."

Unobstructed view

Packer and other experts recommend that Northern California residents bring binoculars, and drive to a hill or ridge top with a view to the horizon, preferably the ocean, unobstructed by trees or buildings.

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"If you can find a place to pull off on Skyline Boulevard, or can pull off the road on Mount Hamilton, those are great places," Packer said.

Tuesday and Wednesday offer a helpful guide: The comet will be visible near a crescent moon. So to find it, focus on the areas around the moon.

"If you are a beginner and you are worried about how you're going to find it, the best thing is to wait until Tuesday," said Andrew Fraknoi, chairman of the astronomy department at Foothill College in Los Altos.

Coastal weather over the next few days is expected to be relatively clear, although fog or other haze can always pose a viewing risk.

"You should try to see it by the end of next weekend if you're interested," Fraknoi added. "It will get fainter as it gets farther from the sun. It is moving outward."

Astronomers often call comets "dirty snowballs." They are essentially collections of rock, dust and ice, with heads that range in size from a few hundred yards across to more than 10 miles across, zooming through space at thousands of miles an hour. They are believed to be remnants from the formation of the solar system, roughly 5 billion years ago, and have sparked awe and wonder since ancient times. Sometimes, they come with great hype and expectation, but fizzle, as Comet Elenin did in 2011 when it disintegrated. Other times, they create amazing visuals, as Comet Hale-Bopp did in 1997, which was so bright it was visible to the naked eye for 18 months -- twice as long as the previous record holder from 1811.

"Predicting comets, especially first-time comets, is very difficult," said Fraknoi. "It's like predicting the stock market."

Lifetime view

Stargazers are hoping that this week's comet show is a warmup for what could be a much bigger event in November.

Then, a comet named ISON will be flying much closer to the sun than Pan-STARRS is now. That comet was discovered last September by two amateur Russian astronomers using an International Scientific Optical Network (ISON) telescope.

If conditions are right, that comet could be visible in daylight, peaking Nov. 28, and has the potential to be a once-in-a-lifetime event.

"We have our fingers crossed," said Packer. "ISON could the best comet you'll see in your lifetime. But you never know with comets. It's hit or miss."

If you're interested: The San Jose Astronomical Association will be holding a beginner astronomy class from 7:15 p.m. to 8:15 p.m. Friday at Hogue Park in San Jose, located near the intersection of South Bascom Avenue and Highway 85. For more information, go to www.sjaa.net.